Field of the Invention
The present invention is directed to a conversion unit for a document scanner which performs a conversion operation that combines either uniformity correction and thresholding or uniformity correction and halftoning (dithering) and, more particularly, to an apparatus and method that accomplishes these operations with a single memory unit and a single comparator by loading the memory unit with either uniformity corrected thresholds or a uniformity corrected halftoning (dither) matrix.
Description of the Related Art
Document scanners exist in a number of different types of devices, facsimile machines, optical character readers, photocopiers etc. in such scanners, a white reference document, mounted in the scanner unit adjacent the document scan window, is conventionally scanned by a light sensor array before the actual document is scanned. The scanning of the reference document is used to create a scale factor (sf) or correction factor or constant for each pixel in the sensor which is used to compensate for aging of the sensor, different background illumination levels, etc. These constants, which are stored in a constant storage as illustrated in FIG. 1, are used to correct or adjust the output of the sensor as the actual document is scanned. The correction is performed using a multiplier 12 to multiply the sensor output, which can be either digital or analog, by the scale factor. This is variously called uniformity correction or shading correction. Once the sensor output is corrected, it is conventionally compared, by a comparator 14, to a single constant threshold, from a threshold storage 16, or to a variable threshold (sometimes called a dither threshold or a halftone threshold) stored in a halftoning matrix storage 18 as selected by a selection unit 20 controlled by the user. The output of the comparator 14 is digital even though the inputs from the selection unit 20 or multiplier 12 can be analog or digital. When the corrected value from multiplier 12 is below the threshold, the output of the comparator 14 is set to a black pixel value (1) and when above the threshold to a white pixel value (0). The image can then be sent to an appropriate processing device for sending the image, if it is a facsimile machine, for reproducing a hard copy of the image, if it is a copier or for performing image recognition operations, if a scanner is for an optical character reader. Examples of prior art devices can be found in U.S. Pat. Nos. 3,800,078, 4,129,853, 4,491,963, 4,524,388, 4,633,314 and 4,980,778.